1. Field of the Invention
The present invention relates to a method of video communication performed between terminal devices connected to a communication network and terminal devices thereof. In particular, the present invention relates to a video communication method and terminal devices having refresh function of coding sequence.
2. Description of the Related Art
Video coding applied to video communication systems follows the coding rule determined in CCITT recommendation H.261 (video coding standard for video communication) which is an international standard. Coding frames generated by the above described video coding include INTRA frames and INTER frames. Each INTRA frame is obtained by "intraframe coding for coding video information of one frame." Each INTER frame is obtained by "interframe predictive coding for coding differential information between one video frame to be coded and one preceding video frame." The above described INTRA frames are generated intermittently. Video information to be transmitted is coded in a predetermined coding sequence so that a plurality of INTER frames may follow one generated INTRA frame.
The above described video coding is premised on an image communication system in which two terminals communicates with each other via a circuit switching network having an assured transmission capacity. A sending terminal sends consecutively coded video information at a fixed rate, whereas a receiving terminal consecutively supplies received video information to a decoding circuit.
However, a conventional video communication scheme adopting the above described inter frame predictive coding has a weakness in that normal reproduction of images from INTER frames cannot be performed when an impermissible gap has occurred in coded video information to be supplied to the decoding circuit in the receiving terminal or when a part of coded video information has been lost during transmission in the network.
Therefore, if video communication using the above described video coding is applied to communication between terminals connected to a packet network for transmitting packeted information, the communication is affected by loss and delay of video packets occurring in the network. Furthermore unadvantageously, once the received image has been disturbed, the receiving terminal cannot output normal video images until the next INTRA frame is sent from the sending terminal.